1. Field of the Invention
The present invention-relates to a surface treating process for stably vapor-depositing an easily oxidizable vapor-depositing material such as aluminum and zinc onto a work such as a rare earth metal-based permanent magnet, a surface treating apparatus and a vapor-depositing material suitable for carrying out such process, and a rare earth metal-based permanent magnet produced by such process.
2. Description of the Related Art
For a rare earth metal-based permanent magnet having a nature that it is liable to be deteriorated by oxidation, for example, it is a conventional practice to form an aluminum film on the surface of the magnet by vapor deposition to prevent the deterioration of the magnet caused by oxidation. A surface treating apparatus, for example, as shown in FIG. 3, is employed for such a surface treating process.
FIG. 3 shows an apparatus for forming a vapor deposited film of aluminum on the surface of a magnet, more specifically, a rare earth metal-based permanent magnet. A single or a plurality of hearths (a vessel for melting a vapor-depositing material) 2, each of which is a melting/evaporating source for evaporating aluminum 10 as a vapor-depositing material, are disposed on a hearth support base 4 risen on a support table 3 in a lower portion of a treating chamber (a vacuum chamber) 1 connected to an evacuating system which is not shown. Two cage-shaped work retaining members 5 each formed of a net-shaped material are disposed side-by-side for rotation about rotary shafts 6 in an upper-portion of the treating chamber 1.
This apparatus is designed, so that rare earth metal-based permanent magnets 30 as works are placed into each of the work retaining members 5, and the aluminum 10 is evaporated from the hearth 2 heated to a predetermined temperature by a heating means (not shown), while rotating the work retaining members 5, thereby forming a vapor deposited film of aluminum on the surface of each of the rare earth metal-based permanent magnets 30 in the work retaining members 5.
However, such surface treating apparatus suffers from (1) a problem that when the vapor deposition process is carried out using such surface treating apparatus under a high partial pressure of oxygen in the treating chamber, the aluminum evaporated from the melting/evaporating source, before reaching the works, is oxidized by oxygen present within the chamber and as a result, an aluminum film having an excellent quality cannot be formed, and (2) a problem that an aluminum oxide film is formed on the surface of molten aluminum within the melting/evaporating source and for this reason, aluminum as the vapor-depositing material is sufficiently not evaporated. If an attempt is made to increase the degree of vacuum for the purpose of reducing the partial pressure of oxygen in order to solve the above problems, it is necessary to carry out an evacuation for a long time. Therefore, if the time taken for the overall processing is supposed to be, for example, 2.5 hours, one hour is required for providing a degree of vacuum equal to or lower than 10−4 Pa, resulting in a problem of a poor productivity.